Chapter 6 summarizes the issue discussed in this thesis and its significance, and the scope of future research
2.3 Characterization techniques
47
Fig. 2.5 Schematic diagram of vacuum deposition system for the evaporation of Au [2], (b) Structure of PSCs showing the evaporated Au metal electrode and the active area.
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2.3.2 Incident photon-to-electron conversion efficiency
Incident photon conversion efficiency (IPCE) measurement was conducted in the wavelength range of 350-850 nm with a 300 W xenon light source and a monochromatic (Asahi Spectra PVL 3300) (see section 1.2.10 of chapter 1)
2.3.3 Ultraviolet-Visible Absorption Spectroscopy
Ultraviolet-visible (UV-vis) absorption spectroscopy is a technique for determining the electronic structure and composition of semiconductor materials based on absorption [4].
When UV-vis light passes through atoms or molecules of semiconductor materials, electrons in the atoms or molecules of the material become excited from a lower energy level to a higher energy level [4,5]. It is worth mentioning that the probability of absorption strongly depends on the material, wavelength, and the distance which the light travels through the material. The absorbance of a material can be expressed using Beer’s Lambert law in the equation below.
where A' is the absorbance, ɛ is the molar extinction coefficient in L mol-1 cm-1, l is the path length in cm, and c is the concentration of the absorbing species in mol/L. The samples in this thesis were measured using a UV-vis spectrophotometer (Shimadzu UV 2450).
2.3.4 Photoluminescence (PL) spectroscopy
Photoluminescence (PL) spectroscopy is a powerful technique for the study and characterization of the electronic structure of materials, and the dynamical processes occurring in materials [6]. It involves measuring the energy distribution of emitted photons after optical excitation. The analyzed energy distribution is used to determine the properties of the material such as defect species, defect concentration, possible stimulated emission, band gap, recombination mechanisms and so on [6].Steady-state PL measurement has been widely employed to investigate the origin of traps in perovskites, passivation effect and charge-hole recombination on the perovskite/ETL or HTL interfaces [7]. The samples in this thesis were measured using a Jasco
NRS-49
5100PL laser Raman spectrophotometer. The excitation wavelength was 325.29 nm, and the detection range was 600-900 nm.
2.3.5 X-ray diffraction
X-ray diffraction (XRD) is a nanocrystallite size measurement technique used to monitor the structure of molecular orientation of materials [4,8]. This technique is highly relevant to the investigation and optimization of charge transport properties of solar cells materials [8]. The technique can measure nanocrystallite size down to as small as 10 Å [4].In X-ray diffraction, a diffraction pattern of the sample is obtained by illuminating the sample with x-rays. The pattern is recorded by detectors [8].Diffraction only occurs when the distance between the adjacent lattices planes follows Bragg’s law for the X-ray to be diffracted from the atomic plane [4]. In the simplest approach, as illustrated in Fig. 2.6, the relationship between the wavelength of the incident X-rays, the angle of incidence and spacing between the crystal lattice planes of atoms can be expressed using Bragg’s law.
Fig. 2.6 Schematic illustration of diffraction processes in XRD measurement.
where n (integer) is the order of interference, λ is the wavelength of the incident X-rays (usually Cu Kα: λ = 1.540562 Å), d is the lattice spacing in nm, and θ is the angle of
d d
θ θ
θ θ
λ λ
λ λ A
A'
C C'
B
B'
Incident X-ray Diffracted X-ray
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incidence in degrees. Note that the diffraction X-rays exhibit constructive interference when the distance between paths ABC and A'B'C' differ by an integer number of wavelength [9]. It is worth mentioning that from the line broadening of the diffraction peaks, the crystallite size D (nm) can be estimated using Scherrer’s equation shown below [4].
where β is the full width at half maximum intensity (FWHM), in radian. θ is the diffraction angle in degrees of the considered diffraction peak, and Kis a shape factor (dimensionless) The value of K is around 0.94 but varies with the actual shape of the crystalline. In this thesis, the XRD patterns were recorded in the 2θ range of 5-60° using an X-ray diffractometer (Rigaku RINT2500V/PC) with Cu Kα radiation (40 kV, 100 mA).
2.3.6 Atomic Force Microscopy
Atomic force microscopy (AFM) was invented in 1986 [11]. It is a powerful technique for nanoparticle size, shape, roughness, and morphology measurement. AFM can be employed for either single composition nanoparticles or mixed nanoparticle composites.
Also, it can be conducted in air and liquid environments. AFM can exhibit a spatial resolution of about 4 nm and has the merit of easy sample preparation. It can give more detailed information regarding particle morphology in the z-direction than the scanning electron microscopy and transmission electron microscopy. However, AFM has the disadvantages of only analyzing small portions of the sample and long data acquisition time. In AFM, the three most important classes of interaction between the probe and the substrate are (i) contact mode, (ii) non-contact mode, and (iii) tapping mode [4].
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Fig. 2.7 Schematic illustration of the working principle of AFM AFM is made up of a probe in the shape of a cantilever with a small tip at its free end, a laser, a four-quadrant photodiode, and a scanner unit (Fig. 2.7). The cantilever bends in response to the force between the tip and the sample [11].The laser beam is focused onto the back of the free end of the cantilever and reflected to the four-quadrant photodiode. This allows detecting the bending of the cantilever with high precision.
When the tip of the cantilever is scanned across the sample surface, the force of interaction between the tip and the sample modifies the static bending of the cantilever (i.e., when the tip is in contact) or the resonance frequency of the cantilever oscillation (i.e., when it is vibrated, and the tip is at a small distance from the surface) [8].In this thesis, contact mode AFM (Keyence VN-8000 viewer and an analyzer) has been used to study the surface roughness and morphology of our samples.
2.3.7 Scanning Electron Microscopy
Scanning electron microscopy is an advanced imaging technique and widely used for the characterization of nanoparticle size, shape, arrangement and degree of agglomeration [4].
Laser Four-quadrant
Photodiode
Cantilever and tip
Sample XYZ stage
Cantilever and tip
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Fig. 2.8 Schematic illustration of SEM set-up
In a typical SEM, an electron gun and multiple condenser lenses produce an electron beam. (Fig. 2.8) The rays of the beam are deflected at a various angle of the optic axis by the sets of electromagnetic scan coils [12]. The objective lens focuses the electron beam to very fine spot (1-5 nm) and scans the sample surface in a raster pattern. When scanning the primary electron beam over the surface of the sample, secondary electrons are emitted from the sample surface and detected, thereby producing SEM image [12].
Also, the backscattered electrons of the electron beam might be detected. The backscattered electron image can be used for contrasting the sample regions. SEM has the advantages of superior lateral resolution and the capability of analyzing a broad range of scales from nanometer range to millimeter range. However, SEM lacks z direction morphology information, and it is also a vacuum technique [8].In this thesis, the morphology of our samples was observed by field emission SEM (FE-SEM; JEOL JSM6335FM, the acceleration voltage of 10 kV). For SEM analysis, the samples were mounted on the sample holder and taped with a conductive tape.
Sample Electron source
Anode Condenser lens
Scan coils Secondary
electron detector
Electron beam
53 2.3.8 Fourier Transform Infrared spectroscopy
The compositional purity of the perovskite film was characterized by Fourier transform infrared spectroscopy (FT-IR) Shimadzu IR-Prestige-21, FT-IR-8400S. The measurements were carried out using samples prepared by the KBr pellet method.
2.3.9 Kelvin Probe Measurement
The work function of WOx and WOx/C60 films deposited on FTO substrates were characterized by Kelvin probe (RIKEN KEIKI FAC-1, A16621-4) vibrating capacitor method. The C60 films were deposited on the FTO/WOx layer by vacuum evaporation in a glove box [13]. The surface potential difference between Au plate and the samples and the work function is evaluated by using Equation 2.4.
where WFs is the work function of the sample surface, WFAu is the work function of the Au (4.9 eV) and VE is the external voltage.
54 2.4 Reference
[1] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, and M. Gratzel, Nature 499, 316 (2013).
[2]S.-G. Park, Study of charge carrier injection and transport in organic light-emitting diodes, http://ir.nul.nagoya-u.ac.jp/jspui/handle/2237/19718
[3] K. Wang, Y.Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, Low-temperature and solution-processed amorphous WOx as an electron-selective layer for perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 755-759.
[4] K. Lu, Nanoparticulate Materials: Synthesis, Characterization, and Processing, John Wiley & Sons, Inc. New Jersey, (2013).
[5] H. Borchert, Solar Cells Based on Colloidal Nanocrystals, Springer Cham Heidelberg New York Dordrecht London, (2014).
[6] G. D. Gilliand, photoluminescence spectroscopy of crystalline semiconductors, Materials Science and Engineering, R18 (1997) 99-400.
[7]Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Origin, and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells, Nat. Commun., 5 (2014) 5784.
[8] F. C. Krebs, Polymer Photovoltaics: a practical approach, SPIE press, USA, (2008).
[9]https://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.html
[10] D. Abou-Ras, T. Kirchartz, U. Rau, Advanced Characterization Techniques for Thin Film Solar Cells, Wiley-VCH Verlag GmbH & Co. KGaA, (2011).
[11] G. Binnig, C. F. Quate, C. Gerber, Atomic force microscope. Phys. Rev Lett. 56 (1986), 930-933.
[12] T. G. Rochow, E. G. Rochow, An Introduction to Microscopy by Means of Light, Electron, X-Rays, or Ultrasound, Plenum Press New York and London, 1978 &1979.
[13] N. Ishiyama, M. Kubo, T. Kaji, M. Hiramoto, Doping-based control of the energetic structure of photovoltaic co-deposited films, Appl. Phys. Lett., 99 (2011), 133301
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